The type of path you want--the path in the filesystem, or through a webbrowser--is usually unambigous depending on your task.
You use HTTP Paths for:
Usually, any file you access through your browser is accessed through an HTTP path.
PHP way to find the HTTP Path of the current file:
In the context of WordPress, you usually need Filesystem Paths only to include() php files.
PHP way to find the Filesystem Path of the current file:
<?php echo dirname(__FILE__); ?>
HTTP: get_settings('home') ? <-- check this. bloginfo have anything relevant?